1. Field of the Invention
The present invention relates to a personal computer system, and more specifically to an interrupt line sharing circuit for a personal computer system, for enabling that a common interrupt line is shared by a plurality of instruments, and a method for sharing the common interrupt line.
2. Description of Related Art
In the prior art, this type of interrupt line sharing circuit has been used in a personal computer system having an edged triggered type interrupt line, for the purpose of enabling that one interrupt line is common to a plurality of instruments and shared by the plurality of instruments, as disclosed in Japanese Patent Application Laid-open Publication No. JP-A-3-191410, which corresponds to U.S. Pat. 5,187,781, the disclosure of which is incorporated by reference in its entirety into this application.
Referring to FIG. 1, there is shown a circuit diagram of the prior art interrupt line sharing circuit disclosed by JP-A-3-191410 and hence U.S. Pat. 5,187,781. In the shown prior art interrupt line sharing circuit, an instrument 101A includes a resistor 200A, a high transistor 213A, a diode 215A and a low transistor 211A connected, in the named order and in their forward direction, in series between a power supply line 107A and a ground line 109A. A connection node between the diode 215A and the low transistor 211A constitutes an output, which is branched to an interrupt line 217A connected through a diode 219A in its forward direction to a common interrupt line 105.
Similarly to the instrument 101A, an instrument 101B includes a resistor 200B, a high transistor 213B, a diode 215B and a low transistor 211B connected, in the named order and in their forward direction, in series between a power supply line 107B and a ground line 109B. A connection node between the diode 215B and the low transistor 211B constitutes an output, which is branched to an interrupt line 217B connected through a diode 219B in its forward direction to the common interrupt line 105.
The common interrupt line 105 is pulled down through a resistor 104, and also connected through a Schmitt trigger circuit 221 to an interrupt request line 102 of a personal computer.
Now, operation of this prior art interrupt line sharing circuit will be described.
In an ordinary condition, the common interrupt line 105 is pulled down to a low level through the resistor 104, so that the interrupt request line 102 is also at the low level. In the instruments 101A and 101B, furthermore, the low transistors 211A and 211B are on and the high transistors 213A and 213B are off, so that the interrupt lines 217A and 217B are also at the low level.
When the instrument 101A generates an interrupt request, the instrument 101A turns off the low transistor 211A and turns on the high transistor 213A, so that the interrupt line 217A is brought to the high level. As a result, the common interrupt line 105 is brought to the high level through the diode 219A, so that the interrupt request line 102 is correspondingly brought to the high level by action of the Schmidt trigger circuit 211. At this time, if the instrument 101B does not generate an interrupt request, the low transistor 211B is on and the high transistor 213B is off. However, the power supply line 107A does not short-circuit with the ground line 109B because of existence of the diode 219B.
Furthermore, if the instrument 101B generates an interrupt request, both the interrupt lines 217A and 217B are brought to the high level. Although the interrupt line 217A is brought to the low level as the result of completion of an interrupt processing for the instrument 101A, since the interrupt line 217B is still at the high level, the interrupt request line 102 is maintained at the high level, so that the personal computer can acknowledge the interrupt request from the instrument 101B.
However, in most of conventional personal computer systems which cannot share the interrupt line, the interrupt request signal is defined as an edge trigger type. Therefore, the above mentioned prior art interrupt line sharing circuit which supplies the interrupt request signal of a level trigger type, cannot be applied to the most conventional personal computer systems.